1. Field of the Invention
The present invention relates to a photolithography mask and in particular to a technique that can provide fast analysis of defects on such a mask.
2. Related Art
FIG. 1 shows a simplified representation of an exemplary digital integrated circuit design flow. At a high level, the process starts with the product idea (step 100) and is realized in an EDA software design process (step 110). When the design is finalized, it can be taped-out (event 140). After tape out, the fabrication process (step 150) and packaging and assembly processes (step 160) occur resulting, ultimately, in finished chips (result 170).
During the fabrication process (step 150), a layout to implement the design is transferred onto a semiconductor substrate. One way to do this is to use the process of optical lithography in which the layout is first transferred onto a physical template, which is in turn used to optically project the layout onto a silicon wafer.
In transferring the layout to a physical template, a mask (usually a quartz plate coated with chrome) is generally created for each layer of the integrated circuit design. This is done by inputting the data representing the layout design for that layer into a device, such as an electron beam machine, which writes the integrated circuit layout pattern into the mask material. In less complicated and dense integrated circuits, each mask comprises the geometric shapes that represent the desired circuit pattern for its corresponding layer. In more complicated and dense circuits in which the size of the circuit features approach the optical limits of the lithography process, the masks may also comprise optical proximity correction features, such as serifs, hammerheads, bias and assist bars, which are sub-resolution, sized features designed to compensate for proximity effects. In other advanced circuit designs, phase shifting masks may be used to circumvent certain basic optical limitations of the process by enhancing the contrast of the optical lithography process.
These masks are then used to optically project the layout onto a silicon wafer coated with photoresist material. For each layer of the design, a light is shone on the mask corresponding to that layer via a visible light source or an ultra-violet light source. This light passes through the clear regions of the mask, whose image exposes the underlying photoresist layer, and is blocked by the opaque regions of the mask, thereby leaving that underlying portion of the photoresist layer unexposed. The exposed photoresist layer is then developed, typically, through chemical removal of the exposed/non-exposed regions of the photoresist layer. The end result is a semiconductor wafer coated with a photoresist layer exhibiting a desired pattern that defines the geometries, features, lines, and shapes of that layer. This process is then repeated for each layer of the design.
As integrated circuit designs become more complicated, it becomes increasingly important that the masks used in photolithography are accurate representations of the original design layout. It is, unfortunately, unrealistic to assume that the electron beam and other machines used to manufacture these masks can do so without error. In the typical manufacturing process, some mask defects do occur outside the controlled process.
A defect on a mask can be defined as anything that is different from the design database and is deemed intolerable by an inspection tool or an inspection engineer. Exemplary defects could include pinhole defects, edge protrusion defects, and geometry bridge defects in the opaque areas of the mask as well as opaque spots and edge intrusion defects in the clear areas of the mask.
FIG. 2 illustrates a simplified system that can inspect a mask for defects. The system includes an inspection tool 200, a stepper image generator 240, and a defect analyzer 290. Inputs to the system include a physical mask 205, a reference description 235, and lithography conditions 265. Reference description 235 can include design layout data (i.e. data that represents a defect free design layout of physical mask 205). In one embodiment, reference description 235 can include a reference image 212 (e.g. an image of a physical mask that was previously inspected and determined to be free from defects).
Inspection tool 200 can include an image acquirer 215, a defect detection processor 225, and a defect area image generator 230. Stepper image generator 240 can include input devices 245 and 255, a mask image simulator 205, and a design image simulator 260. Defect analyzer 290 can include an image comparator 280, a process window analyzer 285, and a performance output device 225.
In this system, inspection tool 200 can inspect physical mask 205 by scanning an image generated by image acquirer 215 for possible defects. This inspection can be carried out by scanning the surface of mask 205 with a high resolution microscope (e.g. optical, scanning electron, focus ion beam, atomic force, or near-field optical microscope) and capturing images of mask 205. For each defect area image that is identified on those captured images, defect detection processor 225 can operate to locate the corresponding area on design layout data 210 and provide this information to input device 255. In one embodiment, the design layout data 210 can be in GDS-II format.
Defect area image generator 230 can generate defect area images of those areas of the mask containing possible defects (as determined by defect detection processor 225). Defect area image generator 230 can provide its defect area image data to input device 245 of stepper image generator 240. Mask image simulator 250 can receive the processed image data from input device 245 as well as lithography conditions input 265 and can generate a defect wafer image 270 as well as simulated process window information.
Note that not all mask defects are important with respect to the desired end result, i.e. an accurate representation of the original design layout on the photoresist material or etched into silicon, because not all mask defects will “print”. Loosely speaking, the printability of a defect is how a defect would impact the outcome of a given photolithography and/or etching process. The importance of printability now becomes apparent, because the goal of defect inspection is to correctly identify a defect to avoid a failed wafer processing.
Because printability of a defect is mainly associated with the stepper exposure, it depends on the particular stepper exposure conditions. Therefore to say a defect is “not printable” means that it has little effect on the expected outcome of a particular stepper exposure, even though it may become “printable” under a different set of stepper exposure conditions. Put in a different way, printability can be highly dependent on lithography conditions 265, because a defect may print under one set of conditions, but not another. Lithography conditions 265 can include: wavelength, numerical apertures coherence factor, illumination mode, exposure time, exposure focus/defocus, and the reflection/transmission characteristics of the defect among others.
Input device 255 of stepper image generator 240, in one embodiment, can receive design layout data 210 corresponding to the defect area from defect detection processor 225 and provide design image simulator 260 with design data representing an area to be simulated that corresponds to the defect area being simulated. Design image simulator 260, also using lithography conditions input 265, can generate a reference wafer image 275 and simulated process window information.
Note that in one embodiment reference image 212 may be provided to input device 255. In this embodiment, design image simulator 260 can then use reference image 212 to generate reference wafer image 275 as well as simulated process window information.
Image comparator 280 of defect analyzer 290 can receive defect wafer image 270 and reference wafer image 275. In one embodiment defect analyzer 290 can include a computer-implemented program that is capable of displaying these simulated images as well as displaying the differences between the two images such that an operator can visually detect any differences.
The simulated process window data from stepper image generator 240 can be provided to a process window analyzer 285 of defect analyzer 290. In one embodiment, process window analyzer 285 can be a computer-implemented program capable of displaying the effect that a potential defect area has on the overall process window of the lithography process as compared to the “perfect” design mask. The simulated process window data from stepper image generator 240 can also be provided to a performance output device 225. In one embodiment, performance output device 225 can be a computer-implemented program capable of determining and displaying the effect that one or more defects have on the overall performance of the integrated circuit for which the physical mask 205 will be used to produce.
If there are no defects, or defects are discovered but determined to be within tolerances set by the manufacturer or end-user, then mask 205 can be used to expose a wafer. On the other hand, if defects are discovered that fall outside tolerances, then mask 205 fails inspection. At this point, a decision must be made as to whether the mask may be cleaned and/or repaired to correct the defects or whether the defects are so severe that a new mask must be manufactured.
Although the system described in reference to FIG. 2 provides accurate mask score results, the simulation performed by stepper image generator 240 can take a significant period of time using valuable system resources. In some cases, faster scoring with less emphasis on accuracy can be useful. For example, mask shops with high volume mask throughput may have less stringent accuracy requirements for such masks. Additionally, fabrication facilities could save critical time by avoiding simulation on (and ultimately repair of) very small (i.e. nuisance) defects on physical masks.
Therefore, a need arises for a fast mask defect scoring technique.